[Ada] Prepare for more efficient signed integer overflow checking

[Arnaud Charlet <charlet at adacore dot com>]

This patch implements expansions for overflow checking to be done
in the gnat_to_gnu translator. Whether expansions are done in the
front end or by Gigi is determined by setting the Backend_Overflow_Checks
flag in the System package. This patch by itself will not change
code generation.

The advantage of expanding overflow checks later is that simpler
checks can be used and implementation is more straightforward because
many Ada specific constructs have already been simplified.

Expansions for addition, subtraction and negation will no longer be
widened, but instead operands are checked before performing the
operation. Multiplication will still be widened if neither operand
is constant and if the type is not a 64-bit type.

Tested on i686-pc-linux-gnu, committed on trunk

2008-07-31  Geert Bosch  <bosch@adacore.com>

	* arit64.c:
	New file implementing __gnat_mulv64 signed integer multiplication with
	overflow checking

	* fe.h (Backend_Overflow_Checks_On_Target): Define for use by Gigi

	* gcc-interface/gigi.h:
	(standard_types): Add ADT_mulv64_decl
	(mulv64_decl): Define subprogram declaration for __gnat_mulv64

	* gcc-interface/utils.c:
	(init_gigi_decls): Add initialization of mulv64_decl

	* gcc-interface/trans.c:
	(build_unary_op_trapv): New function
	(build_binary_op_trapv): New function
	(gnat_to_gnu): Use the above functions instead of
	build_{unary,binary}_op

	* gcc-interface/Makefile.in
	(LIBGNAT_SRCS): Add arit64.c
	(LIBGNAT_OBJS): Add arit64.o

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